Method for making high strength spherical glass bodies



United States Patent 3,389,982 METHOD FOR MAKING HIGH STRENGTH SPHERICALGLASS BODIES Charles W. Schott, Pittsburgh, Pa., assignor to UnionCarbide Corporation, a corporation of New York No Drawing.Continuation-impart of application Ser. No. 154,881, Nov. 24, 1961. Thisapplication Jan. 28, 1964,

er. No. 340,808

9 Claims. (Cl. 6521) ABSTRACT OF THE DISCLGSURE A spherical glass bodyhaving compressive strength in excess of 50,000 p.s.i. when notched to adepth of up to 30% of diameter, resistance to chemical attack anddensity less than 2.65 g./cc. and made by blending particulate glasswith 0.25% to 0.75% acetylene black, mixing the blend with 1% tographite, adding 0.2% to 0.4% water, heating to a temperature such thatthe particulate glass spheroidizes, quenching from a temperature betweensoftening and working and recovering the treated glass bodies.

This invention relates to glass bodies and more particularly to glassspheres and to a method for forming and heat treating such spheres. Thisapplication is a continuation-in-part of my copending application Ser.No. 154,881 filed Nov. 24, 1961, now Patent No. 3,242,032.

There has long been a demand for high strength spheres of glass for useas packing material for chemical reaction towers, as porous loadsupporting beds, as corrosion resistant rollers and for various otherpurposes. There has in addition been a very real need for an undergroundproppant for use in oil wells, gas Wells, and the like to provide a pathof fluid flow in a fractured formation. Various materials have been usedwith varying degrees of success. For example, graded sand has been usedbut it lacks sphericity and has no strength in larger sizes. Walnutshells and aluminum shot have been used but they tend to flatten out.

Glass spheres have heretofore been made in various Ways but the productshave always lacked the property of high strength and have been extremelysensitive to notches and scratches on the glass surface. Such spheres inthe form of solid non-porous spheres have generally had compressivestrengths less than 10,000 lbs. per square inch (p.s.i.) and even whenhigher strengths were achieved such spheres could only be used wherethere was no danger of scratching the sphere surface. Due to theselimitations, the prior spheres were never seriously considered for usessuch as underground proppants.

Accordingly, it is the main object of this invention to provide a glassbody characterized by its sphericity; compressive strengths in excess of50,000 p.s.i.; insensitivity to notches and scratches; and capable ofresisting chemical attack for prolonged periods of time at temperaturesof about 250 F.

It is another object to provide a process for forming and heat treatingglass spheres.

Yet another object is to provide glass spheres which have particularutility as underground proppants in wells such as oil, gas and Waterwells.

3,389,982 Patented June 25, 1968 I have produced a glass sphere whichhas a compressive strength in excess of 50,000 p.s.i. and usually above100,000 p.s.i. and which can maintain this strength even when thesurface has been scratched or notched to a depth of up to about 30% ofthe sphere diameter. These characteristics are unheard of in the art andrepresent remarkable advances over prior spheres.

In my preferred method for producing the above characterized glassspheres lime-soda-silicate glass cullet is blended with from about 0.25%by weight to about 0.75% by Weight of acetylene black. One percent toabout 6% by weight graphite and about .2% to about .4% by weight wateris mixed with the acetylene black-glass cullet blend. This mix is thenheated to a temperature high enough to permit surface tension to formspheres. The heat energy required to produce the temperature desired isprovided from an external energy source such as for example from thecombustion of fuel gases. It should be noted that there is nosubstantial combustion of the mix materials, e.g. carbon. The so-formedglass spheres are quenched from a temperature above the softening pointof the limesoda-silicate glass.

While I prefer to produce and heat treat spheres made fromlime-soda-silicate glass, other glasses such as lead, borosilicate andhigh silica glasses could be employed.

The use of acetylene black has been found to be critical if the glassspheres free from notch sensitivity and having compressive strengths inexcess of 100,000 p.s.i. are to be produced. I have found that thestrengths of spheres made with acetylene black is superior to spheresmade with any of the lampblacks or carbon blacks. In this respect,acetylene black is unique. It is postulated that acetylene black hascertain critical characteristics which not only aid in the forming ofthe glass sphere but also have a beneficial eitect on the heat treatmentof the so-formed spheres. An acetylene black suited for practice of theinvention is made by a process depending on the thermal decomposition ofacetylene at temperatures of about 800 C.

I have also found that the percentage of acetylene black should be keptlow in furnaces which have not been designed to keep air leaks to aminimum. Excessive acetylene black in a so-called loose furnace willmerely burn and aiTord no advantage. Acetylene black in the range of0.25 weight percent to 0.75 weight percent has produced remarkableresults when used with about 1 to about 6% by weight graphite. The totalweight of the carbon is kept extremely low so that the carbon will notcombust and therefore will not act as .a fuel.

The following table illustrates the remarkable improvement in strengthswhen acetylene black is added to a mix prepared from alime-soda-silicate glass cullet.

I- have found that thehigher the-temperature from that for high strengthmaterials, it is necessary to quench from a temperature above thesoftening point of the glass. I have found that glasses may be'airquenched to temperatures approaching 650 C. and then finally quenchedaccording to the practices of my invention and obtain the high strengthscharacteristic of my invention. For example, glasses quenched bydropping through air from 1000 G.- to a temperature of about 675 C. andthen into a quenching medium at room temperature will have a higherstrength than identical spheres which are treated by, conventional airquenching practices.

I have also found that in my method of forming and heattreating glass.spheres the heat treating quenching step may be carried out using anyquenchant having a density less than the glass to bequenched. I havefound that water may be used as the quenching fluid with good recoveriesof product and good compressive strengths. However in order to usewater, certain criticalities must be'observed. To obtain high strengthglass spheres, water must be vaporized and must become a gaseous heattransfer agent. This heat transfer agent must surround the glass sphereand act to keep the liquid (water) from wetting the glass surface.

As a general rule, glass must be softened prior to being quenched intoany medium if high strength is to be realized. In the case of glass andwaterthe glass must be under sufficient tension so that when the glassactually becomes wet and is subjected to the terrific shock that attendsthe wetting, the rupture strength is not exceeded.

For maximum strength and greatest uniformity, a vapor shield shouldenvelop the glass sphere over its entire surface. The sphere and itsvapor shield should be free from any disturbing influences; i.e.,contact with another sphere and/or vapor shield or contact with otherheat absorbing surfaces. After the glass has been cooled to a pointwhere the vapor shield dissolves and the water Wets the sphere surface,contact with other objects will not cause damage.

The more perfect the spherical shape the more uniform will the forces oftension be within the body, primarily because of the uniformity of thevapor shield. Shapes having edges (a cube for example) do not lendthemselves to the protection of the vapor shield.

A recovery approaching 100% can be assured if the above-mentionedrequirements are meteven in water as long as the vapor shield ismaintained during the cooling.

I have developed a successful method for quenching glass spheres inwater. The following detailed description is presented for illustrativepurposes only.

(1) Superheat the glass mix of the type described to form very smooth,highly spherical bodies. I prefer 1000 C. or about 1800 F. for softglasses (container glass, light bulb glass, etc.).

(2) Roll the spheres into water so that the spheres are well spaced andnot touching as they enter the water.

(3) Provide sufficient heat in the spheres so that a vapor shield isdeveloped around the sphere at the time the sphere enters the water.

(4) Maintain the water bath at a specific temperature so as to controlthe heat transfer rate through the vapor shield.

(5) Control water bath temperature so as to control the size of orvolume of vapor shield and consequently the buoyancy of the sphere-vaporshield unit. Water at 200 F. will cause spheres as large as +ie inch tofloat on the surface, supported by a vapor pad. Water at 75 P. willpermit such sized spheres to sink below the water surface surrounded bya vapor shield.

(6) Regulate temperature of the sphere so that it will be plastic, butat the moment of reaching the water surface level it will develop avapor shield covering and protection for the entire sphere surface.

(7) Maintain the depth of water so as to permit free fall of the spheresbeyond the time when the vapor shield dissolves (or is absorbed bysurrounding water).

(8) Regulate all temperatures (water and glass) so as to produce aquench and develop maximum tension within the sphere with a minimum ofresidual heat in the sphere when the vapor shield disappears.

The following example illustrates the type recoveries and strengthsobtainable when glass spheres are made and heat treated using water asthe quenchant according to my invention.

In this run, 8000 gms. of glass cullet sized to 8 x 14 mesh U.S. Seriesand having the approximate composition 73.6% SiO 16.0% Na O, 0.6% K 0,5.2% CaO, 3.6% MgO, and 1.0% A1 0 was blended with 240 gms. graphitepowder, 40 gms. of compression acetylene black and cc. of tap water. Theblended mix was fed to a 6-inch diameter externally heated rotary kiln.The kiln was heated to a maximum temperature of 960 C. The spheresimmediately on leaving the kiln were quenched in ordinary tap water atroom temperature. After washing and drying, the spheres werecompression-tested between 25 R, steel plates at a beam loading speed of0.01 inch per minute. Results are presented below.

Percent rec. 88.75 Strength (p.s.i.):

8 x 12 M 144,250 12 x 20 M 140,660

Compressive strength was estimated according to the formula where L=critical load in pounds at failure D =diameter squared in inches.

The formula only applies to spherical bodies.

The practice of my invention will be further illustrated by reference tothe following example.

Ordinary lime-soda-silicate glass was crushed to produce glass culletfeed of 8 x 14 M. The feed was mixed in the proportion of 2400 glasscullet, 84 graphite and 12 acetylene black. To this mixture was added 9parts water. The blend was externally heated in a rotary kiln to atemperature of about 1000 C. until surface tension caused spheres to heformed. The spheres were discharged into a bath of ethylene glycol atroom temperature. The glass spheres were recovered and tested and foundto have an average compressive strength in excess of 100,000 p.s.i.These tests were made 'by placing the spheres between two hardened steelplates (Rockwell C hardness of 25) and exerting a force on the plates sothat there is essentially point contact on the spheres. The averagecompressive strength was in excess of 100,000 psi.

The glass spheres so produced not only have good strength but have goodsphericity which of course is beneficial to fracture flow capacity whenusing the spheres as well proppants.

Fracture flow capacity of spheres made according to the invention wascompared to the fracture flow capacity of conventional proppants. Theresults of fracture flow capacity tests and other comparative physicaldata are set out in the following table.

TABLE Depth Size, Concentration Fracture Equivalent, U.S. Flow FormationOver burden Type Series Particles Lb./gal. in Capacity Remarks PressureMesh per in. Fracture (md.-ft.)

(p.s.i.)

Sand -20 125 6.0 17,000 Modertatelly embedded, slightly erus e do 8-1264 6.0 8,000 Moderlately embedded, moderately erus 1e Rounded walnutshells 6-8 10 0. 8 20,000 Slightly embedded, slightly Aux Vases Sand.3,000 pancaked.

Glass spheres 6-8 31 5. 9 118,000 Moderately embedded, very slightlycrushed. Sand 4-6 6.0 10,000 Moderately embedded, very severely crushed.Rounded walnut shells. 4-6 5 0.8 25,000 Moderately embedded, slightlypaucaked. Ottawa Sand 10-20 700 Slightly embedded, severely crushed.Dowell Sand 1 8-12 l00 Do. Connell Sand G, 000 Rounded walnut shells8-12 0.8 13, 000 Slightly embedded, moderately sneaked. Aluminum pellets8-12 20 48. 000 p 0. Glass spheres 8-12 44 4. 0 116, 500 SlightlhyEmbedded, very slightly crns e 1 Monolayer pack.

The fracture flow capacity is recorded in millidarcy feet of flow(md.-ft.). This is a well recognized method in the oil industry ofdetermining the efiectiveness of propping agents. It is determined bymultiplying the permeability in darcys by the width of the fracture inwhich the propping agent is placed. High fracture flow capacities meanlarger recoveries from the fractured area and is of utmost significanceto petroleum and gas recovery.

The glass spheres of my invention have other highly desirablecharacteristics. Tests have shown that spheres made by my process aresubstantially insensitive to notch. Compression tests were made onscratched beads made by the prior art and by my method. The prior artscratched spheres had average strengths of 32,300 psi My spheres hadstrengths of about 156,820 p.s.i. A scratch or notch must penetrate atleast of the diameter of my spheres to appreciably effect crushstrength.

The glass spheres of my invention are resistant to chemical attack forprolonged periods at about 250 F.

My glass spheres do not deteriorate with formation of undesirablechemical by-products.

I have found that I may use as the starting material for the preparationof high strength proppants any of the higher melting heat resistingglasses as well as ordinary soft glasses.

I have found that in addition to water and ethylene glycol otherquenching fluids which have a density less than the glass spheres may beused, e.g. aqueous solution of soap, starch and ethylene glycol,triethylene glycol and like materials.

I have found that such glass proppants as here herein described musthave a density below that of about 2.65 gm./ cc. in order that they maybe properly suspended in conventional fracturing fluids which carry theproppant into the strata which is to be supported. These glass proppantsare chemically inert at about 250 F. This is necessary in order toprevent their being chemically attacked and eroded by acid used in somefracturing operations. The pH range encountered in well stimulations mayvary and the proppant must be able to withstand such varying conditions.Since the bottom hole temperatures of oil and gas wells may beconsiderably higher than the surface temperatures the proppant must bephysically stable at temperatures of up to about 250 F. in order tosatisfactorily serve as a proppant. In addition, the proppant to bepumpable must have smooth surfaces of generally spherical shape as wellas high strength.

I claim:

1. A method for forming and heat treating spherical bodies of glasscomprising the steps of (a) blending glass with from about 0.25 to about0.75% acetylene black;

(b) mixing said blend of glass and acetylene black with from about 1 toabout 5% graphite;

(c) adding about .2 to about .4% water of said mix;

((1) heating the so-mixed materials without any substantial combustionof said materials to a temperature above the softening point of theglass so that the glass becomes plastic and its viscosity is reduced topermit surface tension so form spheres;

(e) holding the heated mix at said temperature above the softening pointuntil the softened glass forms spherical bodies by reason of its surfacetension;

(f) quenching these spherical bodies from a temperature between thesoftening point and the working temperature of the glass in a liquidhaving a density lescsl than the density of the material to be quenched;an

(g) recovering a heat treated spherical body having a crush strength ofat least 100,000 p.s.i.

2. A method for forming and heat treating spherical bodies oflime-soda-silicate glass comprising the steps of (a) blending glass withfrom about 0.25 to about 0.75 acetylene black;

(b) mixing said blend of glass and acetylene black with from about 1 toabout 5% graphite;

(c) adding about .2 to about .4% water to said mix;

(d) heating the so-mixed materials without any substantial combustion ofsaid materials to a temperature above the softening point of the glassso that the glass becomes plastic and its viscosity is reduced to permitsurface tension to form spheres;

(e) holding the heated mix at said temperature above the softening pointuntil the softened glass forms spherical bodies by reason of its surfacetension;

(f) quenching these spherical bodies from a temperature between thesoftening point and the working temperature of the glass in a liquidhaving a density less than the density of the material to be quenched;and

(g) recovering a heat treated spherical body having a crush strength ofat least 100,000 p.s.i.

3. A process according to claim 1 wherein said liquid is water.

4. A process according to claim 1 wherein said liquid is ethyleneglycol.

5. A process according to claim 1 wherein said liquid is triethyleneglycol.

7 8 6. A process according to claim 1 wherein said liquid ReferencesCited is aqueous solution of salt. UNITED STATES PATENTS 7. A processaccording t0 claim 1 W fl'e S d liq i 2 4 0 977 2/1949 D i et L 65.21 isaqueous solution of Starch. 5 2,461,011 2/ 1949 Taylor et a1 65-21 8. Aprocess according to claim 1 wherein said l qu d 3,242,032 3/1966 Schott65-21 is aqueous solution of ethylene glycol.

9. A process according to claim 1 wherein said liquid DONALL SYLVESTER,P r lmal'y Exammeris aqueous solution of triethylene glycol. G. R.MYERS, R. L. LINDSAY, Assistant Examiners.

